Learn Russian: Мама читает журнал в комнате.

This is the Prepositional case, which is used to show location (where something is happening). Because we are saying in the room, the preposition в (in) is followed by the Prepositional case. For most feminine nouns ending in -а like комната, the ending changes to -е, making it в комнате.

The word журнал is the object of the sentence. Why didn't its ending change?

You are right that журнал (magazine) is the direct object being read, which means it is in the Accusative case. However, for inanimate (non-living) masculine nouns, the Accusative case looks exactly the same as the basic dictionary form. So, журнал does not change its ending.

The translation says is reading, but can читает also mean reads?

Yes! Russian does not have a separate continuous tense like English does. The single verb читает covers both reads in general and is reading right now. You just use the context to decide which sounds best in English.

How do we know it translates to a magazine and the room if those words aren't in the Russian sentence?

Russian does not have articles like a, an, or the. The English translation just adds them to make the sentence grammatically correct and natural. We usually assume someone is in a specific room (the room), while a magazine might just be any general magazine (a magazine).